Docs: align softmax backend usage with current API

[debashishc]

Summary

• replace stale softmax env-mode docs (FORGE_SOFTMAX_IMPL) with backend-registry usage
• update benchmark docs from --impl to --backend {ref,kernel}
• document strict softmax contract (dim=-1, 2D CUDA, fp16/bf16/fp32)
• refresh kernel status text to reflect current softmax_online wiring

Notes

• docs-only patch (README, DEVELOPMENT, CHANGELOG)
• no runtime/code changes